For many applications in fluid sampling and flow switching, particularly in analytical instrumentation, it is desirable to utilize a flow path as nearly uniform in cross section areas as possible. For this reason, shear seal type valves having a conical or cylindrical valve element having flat plate slider or rotor (valve element) are generally used. Such devices are well known in the art. These valve elements have ports and flow passages which are repositioned for various reasons after rotating or sliding mating sealing surfaces. Leakage is prevented by the sliding seal surfaces. Pressures encountered in some instrumental applications may exceed 5,000 psig for gases or liquids of virtually any composition. Operating temperature requirements may range from lower than -100.degree. C. to more than 300.degree. C.
Sliding seal surfaces, in general, may not be lubricated since lubricants may contaminate the flowing fluids. This contamination is intolerable in valves located in laboratory analytical test instruments. One important design criteria associated with such devices, however, is that leakage must not occur at the sealing surfaces; they must have spring pressure forcing the sealing surfaces together sufficiently to seal at the maximum rated pressure, even though the valve may seldom actually encounter such high pressures. This requires that, at less than maximum pressure operation, the valve is nonetheless able to operate readily at its maximum pressure loading. This results in much faster wear of the sealing surfaces than would be the case if the actual sealing force only slightly exceeded the level required to maintain a sealant that pressure. Additionally, there are applications where a shear seal valve may be exposed to high pressure at relatively lower temperature, and must also be able to operate at elevated temperatures and lower pressures in the same analytical procedure. If a shear seal valve known in the art were operated at its maximum pressure rating and its maximum rated temperature, it would likely be damaged or its lifetime greatly reduced.
The present invention is an apparatus which overcomes these problems by using the pressure of the fluid being controlled by the valve to assist a spring in sealing the valve. This variable loading varies the force on the valve seal surfaces to improve the operating life of the sealing surface while reducing the force required for valve actuation under normal operational conditions. The apparatus dynamically permits operation, without external adjustment, with high pressure/low temperature and low pressure/high temperature fluids in the same device.
The present invention is uniquely able to handle these seal life problems. Additionally, the improved seal loading apparatus makes possible analytical procedures such as direct sampling of high pressure liquids with subsequent programming of system temperature to utilize the high boiling fraction after the sample is injected through this valve, enabling the sample pressure to be reduced.
This disclosure is directed to both a method and apparatus for controlling the sealing force acting on a shear valve. The improved valve includes a chamber with piston pressure actuated to load a shear seal multiport valve element with a mating seal. A piston applies a variable force from the fluid under control to the sealing surfaces. A spring or springs, typically Bellville washers or a coil spring, applies a nominal loading force as a required load to seal the valve at minimum pressures; the force is increased by pressure loading to a maximum rating.
The highest pressure to which the valve is exposed acts through the chamber having the moveable piston element, creating a force added to the spring force to press the valve element against the seal surfaces. The spring has an appropriate minimum force to provide loading to the seal surfaces at the minimum pressure to be encountered. The spring may hold the seal at a pressure such as 1,000 psig for a 7,000 psig maximum pressure rated valve. The additional force exerted by the flowing fluid (at up to 7,000 psi) increases as the fluid pressure increases. The increased force seals up to but not beyond a selected maximum pressure, the predetermined maximum pressure for the valve. Beyond this crossover point, the valve controllably leaks at the seal to relieve pressure to prevent damage to the seal surface and to other flow system components.